Abstract: Computer simulation of the transversal spatial effects in CdS films at the thermo optical bistability is completed using Ulrbach's theory about red shift of absorption spectra curve of semiconductor excited by light and Eiry's theory about absorption F-P resonator.The simulation is consistent with the experimental results.Maximum density (104/cm3) of elements on OB matrixes of CdS films (thickness 1 μm) at room temperature is calculated.